The present invention relates to an image forming apparatus such as a full-color copying machine or color printer.
A known example of a conventional image forming apparatus for outputting a color image is a so-called 4-drum tandem type full-color copying machine constituted by parallel-arranging, along a convey belt, four image forming units for forming toner images of yellow (Y), magenta (M), cyan (C), and black (B) on the basis of color-separated image signals.
The image forming unit of each color has a photosensitive drum in rolling contact with the convey belt, a charging device for charging the drum surface to a predetermined potential, an exposure device for exposing the drum surface to form an electrostatic latent image, a developing device for supplying toner to develop the electrostatic latent image on the drum surface, and a transfer device for transferring the developed toner image to a recording sheet which is attracted and conveyed by the convey belt. The recording sheet attracted by the convey belt is conveyed through the four image forming units, and toner images of the respective colors are transferred over each other on the recording sheet. Then, the recording sheet is fed to a fixing device where the toner images of the respective colors are fixed to the recording sheet to form a color image.
In the 4-drum tandem type full-color copying machine using a DC motor for driving the photosensitive drums, the four photosensitive drums have the same diameter, a multi-pulse encoder is used to increase the rotational precision, and constant angular speed control using the pulse signal is employed.
The photosensitive drum is shorter in service life than the whole apparatus, and must be exchanged every several ten thousand prints. For this purpose, the photosensitive drum has an arrangement (mechanism) detachable via a coupling connected to a DC motor serving as a driver for the photosensitive drum. Although the axial shift of this mechanism must be minimum, the number of separation components increases because of the detachable arrangement, and the axial shift greatly increases owing to the size difference or play between components. The axial shift causes variations in rotational angle (peripheral speed) of the periodicity (appearing as a sine wave) in one rotation of the photosensitive drum.
In the 4-drum tandem type full-color copying machine, therefore, the periods of rotational variations of the respective photosensitive drums differ from each other, and large image misalignment (density difference or rainbow noise) occurs.
When DC motors are used as drivers for the respective photosensitive drums, the photosensitive drums are not controlled in activation or stop. Even if the periods of rotational variations of the photosensitive drums are temporarily adjusted, peripheral speed variations of the photosensitive drums are out of phase.
It is an object of the present invention to solve a problem that the periods of rotational variations of respective photosensitive drums differ from each other to generate large image misalignment in a 4-drum tandem type full-color copying machine, and lock the phases of the periods of rotational variations of the photosensitive drums, thereby minimizing and reducing image misalignment.
To achieve the above object, the present invention provides an image forming apparatus characterized by comprising
a convey portion for conveying a recording medium,
a plurality of image forming units which are aligned on the convey portion, have rotational photosensitive drums, and transfer images of different colors to the recording medium conveyed by the convey portion,
output means for outputting a pattern representing speed changes in one rotation of the photosensitive drums of the respective image forming units,
setting means for setting correction values based on phase differences of the speed changes in one rotation of the photosensitive drums that are determined from an output from the output means, and
correction means for locking phases of the speed changes in one rotation of the photosensitive drums by increasing or decreasing rotational speeds of the photosensitive drums on the basis of the correction values set by the setting means.
The present invention provides an image forming apparatus comprising
a convey portion for conveying a recording medium
a plurality of image forming units which are aligned on the convey portion, have rotational photosensitive drums, and transfer images of different colors to the recording medium conveyed by the convey portion
output means for outputting a pattern representing speed changes in one rotation of the photosensitive drums of the respective image forming units
detecting means for detecting phase differences of the speed changes in one rotation of the photosensitive drums, outputted from the plurality of image forming units
setting means for setting correction values based on results detected by the detecting means and
correction means for locking phases of the speed changes in one rotation of the photosensitive drums by increasing or decreasing rotational speeds of the photosensitive drums on the basis of the correction values set by the setting means.
The present invention provides an image forming apparatus comprising
a convey portion for conveying a recording medium
a plurality of image forming units which are aligned on the convey portion, have rotational photosensitive drums, and transfer images of different colors to the recording medium conveyed by the convey portion
creating means for creating a pattern of a registration mark for each of the different colors
generating means for generating a toner image of the pattern of the registration mark for each of the different colors, created by the creating means, to the convey portion by the respective image forming units;
a detector, provided along the convey portion on a downstream of the image forming unit, for detecting the toner image of the pattern of the registration mark for the respective colors generated onto the convey portion
measurement means for measuring speed changes in one rotation of the respective photosensitive drums, based on the pattern of the registration mark for the respective colors, detected by the detector
setting means for setting correction values based on results detected by the detecting means and
correction means for locking phases of the speed changes in one rotation of the photosensitive drums by increasing or decreasing rotational speeds of the photosensitive drums on the basis of the correction values set by the setting means.